1. Field of the Invention
This invention relates to a dry developing apparatus designed for use in electrophotographic copying machines, and more particularly to an improved dry developing mechanism of the type recited, which is capable of accurately detecting the situation when the developer in the developer tank or in the developing unit in said mechanism has been decreased to a less than necessary amount.
2. Description of the Prior Art
Generally, in electrophotographic copying machines using a dry developing system, development is practiced by applying a powder developer after forming electrostatic latent images on a sensitive medium. Various dry developing systems have been devised, such as the magnetic brush system, the cascade system, the furbrush system or the powder cloud system, and in any of these systems, the powder developer is supplied or replenished from a hopper-like developer tank into the developing station as required. Also, the developer tanks used in these dry developing mechanisms may employ a wide variety of systems as occasion demands, such as for example a system in which the developer is supplied by dropping it from bottom opening of the developer tank with rotation of a roller having a rugged surface and provided in said opening, a transfer supply system in which the powder developer is transferred axially by rotation of a screw roller provided in the tank bottom, or a system in which the powder is dropped in accordance with reciprocation of a bottom member provided in the bottom opening of the tank.
It is important for consistently obtaining stabilized good-quality copied images to provide means for detecting the situation of shortage or insufficiency of developer in the developer tank and suspending the operation of the device which urges replenishment of the developer. The present invention is to provide a device which is capable of accurately and positively detecting such situation of shortage of developer whenever such situation is reached during operation of the copying machine.
In the electrophotographic copying machines of the liquid developing system, the residual quantity of the developing solution could be easily and accurately detected by using for instance a float means designed to determine the liquid level in the tank or by using a transparent liquid tank so as to determine the difference in the quantity of transmitted light. However, in the systems using powder developer, these methods are unable to correctly detect the residual amount of developer in the developer tank because the powder would adhere to the tank wall surface to intercept the transmitted light and also because the powder developer does not always decrease evenly in the tank. That is, there often occurs a phenomenon that the toner particles attached to the tank wall surface remain stuck thereon and are not used. There has heretofore been developed no satisfactory device which is inexpensive and capable of correctly and reliably detecting the residual quantity of powder developer in the dry type electrophotographic copying machines.